[pkg] Use pytest

tox.ini

@@ -3,8 +3,8 @@
 
 [testenv]
 sitepackages = True
-commands =
-  {envpython} -m unittest discover -s test -b
+deps = pytest
+commands = {envpython} -m pytest {posargs:{toxinidir}/test}
 
 [testenv:flake8]
 skip_install = true